As ball end mills with long lifespan that are used for manufacture of various kinds of components in the automobile industry, the electronic industry, or the like and that can cut high-hardness metal dies with high efficiency, multi-flute ball end mills that have three or more ball edges and are made of cemented carbide are widely used.
PTL 1 discloses a ball end mill in which each of ball edges is constituted with a first cutting portion and a second cutting portion, the curvature radius of the first cutting portion is 0.025D or more and 0.10D or less with respect to the external diameter D, and the curvature radius of the second cutting portion is larger than the curvature radius of the first cutting portion. However, in the ball end mill of PTL 1, gashes in the vicinity of a rotation center between the respective ball edges are small. Thus, clogging of chips during high-feed rough machining cannot be prevented.
PTL 2 discloses a ball end mill in which three or more ball edges are provided at a tip and a center web portion is provided at an axial center portion. However, in the ball end mill described in PTL 2, the degree of curvature of ball edges is small. Therefore, there is a concern that the cutting resistance during high-feed rough machining may increase markedly and the cutting performance may degrade.
PTL 3 suggests a ball end mill which has three or more ball edges and in which, in order to solve a shortage of chip pockets in the vicinity of a rotational axis to prevent occurrence of chip clogging, thinning is performed on lands of the respective ball edges and the respective ball edges are cut out in the vicinity of a rotation center. However, in the ball end mill described in PTL 3, a cutting operation cannot be performed by the vicinity of the rotation center. Therefore, there is a high possibility that the machined face quality of a work material may deteriorate in high-feed rough machining.
PTL 4 discloses a ball end mill in which the radial rake angle of ball edges and peripheral cutting edges may be a negative angle. However, in this ball end mill, the degree of curvature of the ball edges is small. Therefore, there is a concern that the cutting resistance during high-feed rough machining may increase markedly and the cutting performance may degrade.